The present invention relates to a laser diode assembly having a structure wherein a laser diode device is jointed to a heatsink with a submount in between or directly.
More specifically, the invention relates to a laser diode assembly comprising a construction wherein in assembly, solder overflow from a solder joint face between a laser diode device and a submount or a heatsink is inhibited, and overflowing solder does not form bulb-shaped solder residue on the laser diode device, the submount, or the heatsink.
In a laser diode device, when electrical energy is converted to optical energy, energy loss is generated, and such energy loss is converted to heat energy. This heat energy raises temperature of the laser diode device, and has an unfavorable effect on laser characteristics.
In order to immediately diffuse such heat energy generated in the laser diode device, and reinforce mechanical strength of the laser diode device, generally, a so-called laser diode assembly, wherein a substrate side of a laser diode device is jointed to a heatsink directly or with a submount in between through a solder layer is manufactured as a product.
Specifically, as shown in FIG. 7, in a conventional laser diode assembly 10, a laser diode device 12 is jointed to a submount 16 through a first solder layer 14, and the submount 16 is jointed to a heatsink 20 through a second solder layer 18. The laser diode device 12 comprises a lamination structure including a light-emitting layer on a semiconductor substrate. There is a structure wherein the laser diode 12 is directly jointed to the heatsink 20 without the submount 16.
The heatsink 20 is provided with an electrode pad 24 with an insulating plate 22 in between. The electrode pad 24 and an electrode of the laser diode device 12 are connected (wire bonded) through a wire 26. That is, the laser diode device 12 is connected to outer equipment through the electrode pad 24.
The laser diode device 12 is, for example, a GaAs edge-emitting type laser diode device formed on a GaAs substrate. In FIG. 7, an end face opposite of the electrode pad 24 side is an emitting end face. Number of the laser diode device 12 can be one as shown in FIG. 7 or plural. Some of the laser diode device 12 are arranged in the shape of an array.
The submount 16 is a part to reinforce mechanical strength of the laser diode device 12 and is a metal plate made of, for example, silicon carbide (SiC) or an alloy of copper (Cu) and tungsten (W). The heatsink 20 is often made of a copper plate.
The first solder layer 14 to joint the laser diode device 12 and the submount 16 is, for example, a gold-tin (Au/Sn) solder layer. The second solder layer 18 to joint the submount 16 and the heatsink 20 is, for example, an indium (In) solder layer.
When such a laser diode assembly 10 is fabricated, first, the submount 16 which lays the laser diode device 12 on a gold-tin solder plating layer formed on a joint face of the submount 16 is forwarded into a reflow oven, and heated to a melting temperature of the gold-tin solder or higher, for example, 300° C. to melt the gold-tin solder plating layer.
Next, the submount 16 placing the laser diode device 12 is taken out from the reflow oven, and cooled. The melted gold-tin solder plating layer is thereby solidified to become the first solder layer 14, and the submount 16 and the laser diode device 12 are jointed.
Subsequently, indium metal vapor is deposited on a joint face of the heatsink 20 to obtain an indium deposition layer. The submount 16 to which the laser diode device 12 is jointed is laid on the indium deposition layer. Then, the resultant is forwarded into the reflow oven, heated to 156° C. or higher which is a melting temperature of the indium solder, for example, 180° C. to melt the indium deposition layer. Next, the heatsink 20 placing the submount 16 is taken out from the reflow oven and cooled. The melted indium deposition layer is thereby solidified to become the second solder layer 18, and the heatsink 20 and the submount 16 are jointed.
In the conventional laser diode assembly 10, there is an example wherein the second solder layer 18 is made of lead-tin solder instead of indium, and an example wherein the first solder layer 14 is made of lead-tin solder instead of gold-tin solder.
In Japanese Unexamined Patent Application Publication No. H06-350202, a semiconductor light-emitting device having a construction similar to the foregoing laser diode assembly 10 is disclosed.
When the laser diode device is jointed on the submount, or the submount is jointed on the heatsink, even if the same amount of solder is provided every time, a part of excessive solder overflows from the joint face, since surface state of the joint faces of the laser diode device, the submount, or the heatsink is subtly changed.
As shown in FIG. 8, the overflowing solder is swollen in the shape of a bulb, and remains at edges of the joint area, particularly at edges of the laser diode device. This phenomenon that solder is swollen in the shape of a bulb is significant particularly when flux is not used.
The solder residue may become an obstacle in subsequent mounting of the laser diode assembly, or have an adverse effect on laser characteristics. In particular, if the bulb-shaped solder residue is formed on the emitting end face side of the laser diode device when the laser diode device is jointed on the submount or the heatsink, it is not preferable for the laser characteristics.